Brean Capital analyst Gene Mack said in a note dated Sept. 5that Astex should have rejected an offer below $13 per share asit did not reflect the royalties from its drug Dacogen and itsportfolio of drugs partnered with companies including NovartisAG.

Sarissa questioned the timing of a deal that comes beforethe release of key data from a cancer drug, codenamed SGI110,expected as early as December.

The firm, which disclosed a 5 percent stake in Astex, raisedconcerns over Otsuka's expectation of offering greatercompensation opportunities and incentives to the U.S. company'ssenior management.

In an open letter to Astex shareholders, Sarissa said itbelieved that Astex did not contact all potential bidders and itwas reaching out to companies it believed had been left out ofthe bidding.